1. Field of the Invention
The present invention relates to a method for the manufacture of plastic containers, for example, containers for margarine, and especially to an improved method for the thermoforming of such containers from polyethylene or polypropylene.
2. Description of the Prior Art
Methods for the manufacture of plastic containers are known in the art. In particular, in U.S. Pat. No. 3,602,946, issued Sept. 7, 1971, G. E. Curetti, et al. describe a method which involves the formation of a diaphragm of plastic material in a transfer ring with the periphery of the diaphragm being retained in an internal annular groove of the ring. The ring containing the diaphragm is applied to a moulding station where the diaphragm is moulded into a container. The container is then severed from the plastic material retained in the groove, the annulus of plastic material so formed being discharged to waste or re-used.
In U.S. Pat. No. 4,005,164, issued Jan. 25, 1977, B. Proctor describes a method for the manufacture of plastic containers with integral rims in which plastic material is compressed into an annular groove in a transfer ring. The material in the annular groove forms the integral rim of the container on moulding. In the process described by Proctor an annulus of plastic material is not formed.
The method and apparatus described by Proctor have had significant commercial success with a number of thermoplastic materials, for example, polystyrene. However, containers manufactured from polyethylene or polypropylene have been found to be capable of substantial improvement, especially with regard to the characteristics of those containers, e.g., aesthetics and uniformity of the thickness of the walls of the containers.
A method for the manufacture of plastic containers from polyethylene or polypropylene so as to obtain containers of improved quality has now been found.